Field of the Invention
This invention relates to liquid silicone compositions curable to rubber. The cured silicone rubber provides a unique combination of properties including low compression set and unprimed adhesion to a variety of substrates.
Liquid silicone compositions curable to rubber based upon vinyl-containing polydiorganosiloxane polymer, organohydrogensiloxanes containing hydrogen on silicon groups, and platinum group containing catalyst are well known in the art of silicone elastomer compositions. Such compositions are also well known for their reluctance to adhere to substrates against which the composition is cured. Many additives have been developed in attempting to obtain good adhesion of such compositions to substrates. When these additives are used to obtain adhesion, the compression set of the composition increases to an unacceptable level for applications such as gaskets which need to have a relatively low compression set in order to function properly.
U.S. Pat. No. 3,527,655, issued Sep. 8, 1970, teaches a fluid, curable silicone elastomer composition which bonds tightly to surfaces contacting it during cure comprising a vinyl-substituted polysiloxane, an organohydrogen polysiloxane, a platinum compound catalyst, and a cohydrolyzate of vinyl trichlorosilane and vinyltriethoxysilane.
Use of a primer composition rather than an additive is taught in U.S. Pat. No. 3,794,556, issued Feb. 26, 1974. The primer is a mixture of an alkoxy silicon compound, titanate, and an organosilicon compound having trifluoropropyl radicals and hydrogen or triacetoxy radicals. Use of a primer causes at least a two-step process in bonding the silicone rubber to a substrate.
U.S. Pat. No. 3,892,707, issued Jul. 1, 1975, teaches a composition comprising a diorganovinylsilyl-terminated diorganopolysiloxane, an alkenyl isopropenoxysilane, an organohydrogenpolysiloxane, and platinum compound catalyst which gives strong bonding to the surface of a substrate to which it is in contact. There is no indication as to the compression set of the material.
U.S. Pat. No. 3,960,800, issued Jun. 1, 1976, teaches a reaction product prepared by mixing an organosiloxane and an acetoxysilicon compound under anhydrous conditions, either one or both of which has a silicon-bonded vinyl radical. This reaction product when mixed with an alkyl silicate, an organotitanate, and organic solvent forms a primer solution for use on substrates to which silicone elastomers are to be applied.
A curable organopolysiloxane composition which is self-adhering is taught in U.S. Pat. No. 4,082,726, issued Apr. 4, 1978. The adhesion additive is an organosilicon compound having at least one silicon group having attached to the silicon atom one monovalent hydrocarbon group containing at least one epoxy group and two alkoxy groups and at least one lower alkenyl group or hydrogen atom bonded to silicone atoms which are in the same molecule.
A self-adhering composition is taught in U.S. Pat. No. 4,087,585, issued May 2, 1978, wherein the additional ingredients to provide adhesion are a polysiloxane having at least one silicon-bonded vinyl radical and at least one silicon-bonded hydroxyl radical, and an epoxy-containing, alkoxy silane.
U.S. Pat. No. 4,163,081, issued Jul. 31, 1979, teaches use of a vinyl-containing amidosiloxane to give improved adhesion to many substrates.
A composition having improved adhesion is taught in U.S. Pat. No. 4,245,079, issued Jan. 13, 1981. The composition comprises an organopolysiloxane having at least two alkenyl groups bonded to silicon atoms in which the silicon atom is attached to a monovalent hydrocarbon group containing a trialkoxysilyl group, an epoxy group or an ester group, an organohydrogenpolysiloxane and a platinum catalyst. A self-bonding silicone rubber composition comprising a vinyl-terminated polysiloxane polymer, a hydride siloxane crosslinking agent, a platinum catalyst and as the self-bonding additive, the partial hydrolysis product of an aliphatically unsaturated hydrolyzable alkoxy silane is claimed in U.S. Pat. No. 4,311,739, issued Jan. 19, 1982.
An adhesion promoter which is useful in platinum-catalyzed hydrosilation reaction cured polyorganosiloxane compositions is taught in U.S. Pat. No. 4,737,562, issued Apr. 12, 1988. The adhesion promoter is a combination of 1,2-bis(trialkoxysilyl) ethane, an organosilicon compound containing at least one hydroxyl group or hydrolyzable precursor and at least one group that reacts at ambient temperature with the alkoxy groups of the silylethane, and a catalyst for the reaction of the adhesion promoter ingredients.
U.S. Pat. No. 4,946,878, issued Aug. 7, 1990, teaches the use of a mixture of diorganoalkenylsiloxy-terminated polydiorganosiloxanes where the alkenyl radical in one of the diorganosiloxanes is a radical which is terminally unsaturated and contains at least 4 carbon atoms to give a faster cure rate than when the radical is a vinyl radical.
European Patent Application 384352, published Aug. 29, 1990, teaches an adhesive promoter comprising either a mixture or reaction product of a carboxylic acid ester containing an acryloxyloxy or methacryloxyloxy group and a hydroxyalkyl group and an organosilicon containing silicon-bonded vinyloxy or a substituted vinyloxy group.
A self-bonding composition is taught in U.S. Pat. No. 5,006,580, issued Apr. 9, 1991, in which the additives are a carbonyl complex of a Group VIB element and a compound containing a silicon-bonded alkoxy group which is selected from the group consisting of silanes or siloxane oligomers.
A silicone rubber adhesive described in U.S. Pat. No. 5,023,288, issued Jun. 11, 1991, contains wet-method hydrophobicized reinforcing silica, acryl-functional silane coupling agent or methacryl-functional silane coupling agent, epoxy-functional coupling agent, and partial allyl ether of a multivalent alcohol as the adhesion producing ingredients.
A self-bonding elastomer in a one-part organosiloxane composition which cures by a platinum-catalyzed hydrosilation reaction is taught in U.S. Pat. No. 5,082,894, issued Jan. 21, 1992. Long time storage stability, short cure time, and cohesive bonding to a variety of substrates are achieved by including an acetylenic alcohol, an alkoxysilane containing an epoxy group, and a polyorganosiloxane containing an average of at least one silicon-bonded hydroxyl group and at least one silicon-bonded vinyl radical per molecule.
U.S. Pat. No. 5,110,845, issued May 5, 1992, teaches use of calcium hydroxide in order to reduce compression set.
One of the methods of sealing valve covers, rocker covers, oil pans, and such in automotive engines is through the use of cured-in-place gaskets. These gaskets are produced by extruding a bead of sealant material onto the part to be used and curing the material in place. The material must bond to the substrate. Because the part is then clamped onto the engine block, the sealant material must have a low compression set because the purpose of the gasket is to prevent leaks at the joint formed between the part and the engine. The composition used as the sealant material must be heat stable, have good compression set when cured, and adhere to the substrate to which it is applied.